Quantum mechanical modeling for chemical kinetics of interstellar reactions and spectroscopic signatures of interstellar species

Abstract

In the first part of this work we investigate the possible reaction pathways for the gas phase formation of cyclopropenone and propylene oxide in Sagittarius B2. The possible formation reactions are proposed with the assumption that only two body collisions are probable at the low temperature and densities associated with the cold interstellar gas clouds. The likelihood of each proposed formation reaction is deduced based on two criteria: 1) the lack of any activation barrier, 2) the possibility of internal and external energy dissipation. The barrierless exothermic reactions must release the excess energy to keep the newborn interstellar molecule from breaking apart. The rate constants of possible reactions satisfying the two criteria are calculated using the collision theory. In the case of non-adiabatic reactions accompanied by the change of spin multiplicity, the probability of transition between two electronic states is taken into account. We demonstrate that the spin-allowed barrierless reaction of cyclopropenylidene and hydroxyl radical is kinetically and thermodynamically favorable and has a relatively large rate constant. The hydrogen atom acting as a side product can carry the excess energy in this reaction away. Also, the spin-forbidden reactions of propene and atomic oxygen and ethanal and methylene radical can contribute to the interstellar formation of propylene oxide. In these two reactions, radiative association is proposed to be responsible for the excess energy dissipation.In the second part, we investigate the electronic transitions of C60+ fullerene and C60+-He, which are believed to be responsible for two strong diffuse interstellar bands (DIBs) at 9632.7 Å and 9577.5 Å. The following hypotheses for the origin of two electronic transitions responsible for these DIBs are tested: 1) transitions to two differentiexcited states of C60+ (D5d isomer), 2) transitions to two spin-orbit components of an excited state, 3) transition to two components of pseudo Jahn-Teller distorted C60+, and 4) two transitions in the distorted C60+-He complex. Our results demonstrate that the He atom perturbs the electronic structure of C60+ and gives rise to two electronic transitions with the energy gap and the intensity ratio comparable to the experimental values. Therefore, we propose that the C60+-He complex, instead of C60+, could be responsible for the two DIBs.